Technologies described in Patent Document 1 and Patent Document 2 mentioned below are already known as the control device described above. The technologies described in Patent Document 1 and Patent Document 2 are configured to perform starting control for an internal combustion engine in which a first engagement device is caused to transition to a slipping engagement state to raise the rotational speed of the internal combustion engine using a rotational drive force of a rotary electric machine in the case where a starting request for the internal combustion engine is made when the first engagement device is in a disengaged state and a second engagement device is in a direct engagement state.
In order to shorten the starting time for the internal combustion engine, the technology according to Patent Document 1 is configured to start transition of the first engagement device from the disengaged state to the slipping engagement state before the second engagement device is caused to transition from the direct engagement state to a slipping engagement state.
In the technology according to Patent Document 1, in order that torque transferred from the rotary electric machine to the side of wheels is not reduced even if slip torque with a magnitude of the transfer torque capacity of the first engagement device is transferred from the rotary electric machine to the internal combustion engine side when the first engagement device is caused to transition to the slipping engagement state, a target transfer torque capacity for the first engagement device is added to target torque for the rotary electric machine to compensate for a reduction in torque due to the slip torque in a feedforward manner.
In the technology according to Patent Document 1, however, in the case where there is a compensation error for the slip torque of the first engagement device, a torque shock due to the compensation error is transferred to the side of the wheels via the second engagement device which is in the direct engagement state, which may give a driver an uncomfortable feeling.
Meanwhile, the technology according to Patent Document 2 is configured to set a target rotational speed to perform rotational speed control for a rotary electric machine even in the case where a method of starting an internal combustion engine in which a second engagement device is not controlled to a slipping engagement state is selected. Although the technology according to Patent Document 2 does not disclose the details of the configuration for setting a target rotational speed, it is considered that the rotational speed control acts in the direction of reducing a torque shock caused when a first engagement device is caused to transition to a slipping engagement state. In Patent Document 2, however, the first engagement device is caused to transition from the slipping engagement state to a direct engagement state with the second engagement device kept in a direct engagement state, and therefore there is a limit to the extent to which transfer of a torque shock generated when the first engagement device is caused to transition from the slipping engagement state to the direct engagement state to the side of the wheels is suppressed.
Thus, the inventor considered suppressing a torque shock caused when the first engagement device is caused to transition from the slipping engagement state to the direct engagement state by bringing the second engagement device into the slipping engagement state. However, the inventor noticed that if it is attempted to cause the second engagement device to transition to the slipping engagement state while the rotational speed control is performed, a rise in rotational speed of an engagement member of the second engagement device on the rotary electric machine side may be suppressed through the rotational speed control, and that in the case where it is attempted to determine that the second engagement device is brought into the slipping engagement state on the basis of a difference between the rotational speeds of engagement members of the second engagement device, there may be a delay in timing to determine that the second engagement device has been brought into the slipping engagement state which leads to elongate the starting time for the internal combustion engine.